Justices decide firmly for privacy in their first ruling on government use of digital technology to monitor people.

The Supreme Court confronted for the first time the government's growing use of digital technology to monitor Americans and ruled strongly in favor of privacy.

The court said the Constitution generally barred the police from tracking an individual with a GPS device attached to a car unless they were issued a warrant from a judge in advance. But the ruling could limit a host of devices including surveillance cameras and cellphone tracking, legal experts said.

"I would guess every U.S. attorney's office in the country will be having a meeting to sort out what this means for their ongoing investigations," said Lior Strahilevitz, a University of Chicago expert on privacy and technology.

Even the justices who most often side with prosecutors rejected the government's view that Americans driving on public streets have waived their right to privacy and can be tracked and monitored at will. At least five justices appeared inclined, in the future, to go considerably beyond the physical intrusion involved in putting a GPS device on a car and rule that almost any long-term monitoring with a technological device could violate an individual's right to privacy.

Until now, prosecutors and police have believed as long as they were tracking a person who was out in public, they could use GPS devices, cellphone tracking, facial recognition cameras or computer data mining to gather a dossier on an individual without a search warrant. A majority of the justices aggressively rejected that idea Monday.

Although the justices agreed on the outcome, they quarreled over how to approach the issue and how far to go.

Five justices, led by Antonin Scalia, said the police violated the 4th Amendment's ban on unreasonable searches when they attached the device to a vehicle's bumper and monitored its movements.

Justice Samuel A. Alito Jr., whose opinion was joined by three others, cited "dramatic technological change" that has made it "relatively easy and cheap" for agents to secretly monitor people and gather huge amounts of information. Justice Sonia Sotomayor agreed, saying "awareness that the government may be watching chills" freedom.

None of the opinions set a strict limit on searches, but they signaled the court was determined to limit officials' power to monitor individuals, at least when there is no "probable cause" to believe the individuals have committed a crime.

The case before the court arose when Antoine Jones was charged with running a drug-dealing operation in the Washington area, based in part on data gathered from tracking his Jeep.

By a 9-0 vote, the justices ruled it was unconstitutional for the police to attach a small GPS device to his bumper and track his car for a month. The tracking data helped convict Jones of running a drug-dealing operation.

It is rare when a drug criminal wins in the conservative-leaning high court, but the GPS case concerned whether the modern state had unlimited power to track and monitor its citizens, evoking the specter of George Orwell's "1984."

"Society's expectation has been that law enforcement agents and others would not — and indeed, in the main, simply could not — secretly monitor and catalog every single movement of an individual's car for a very long period," Alito wrote, adding that such a search "surely crossed" the constitutional line. Justices Ruth Bader Ginsburg, Stephen G. Breyer and Elena Kagan agreed with Alito.

Sotomayor joined Scalia's majority opinion, but in a concurring opinion she made clear that she also agreed with much of Alito's broader view that the court must limit the government's use of tracking technology.

Scalia's view protects the "constitutional minimum" that the government may not trespass on private property, she wrote. But she said these devices could "make available at a relatively low cost such a substantial quantum of intimate information about any person who the government, in its unfettered discretion, chooses to track." Moreover, she said, "the government can store such records and efficiently mine them for information years into the future." The government cannot be free to use "a tool so amenable to misuse," she said, particularly in light of the 4th Amendment's aim "to curb arbitrary exercises of police power."

Scalia did not foreclose a future decision that tracking through "electronic means" is an "unconstitutional invasion of privacy," but said there was "no reason for rushing forward" to resolve that issue now. Chief Justice John G. Roberts Jr.and Justices Anthony M. Kennedyand Clarence Thomas also joined the opinion.

The ruling in U.S. vs. Jones left many questions unanswered. Alito and Sotomayor did not say when electronic tracking goes too far.

Privacy experts hailed the opinion as a welcome surprise and a good portent for the future.